Non-oxidizable coating

ABSTRACT

A substrate is coated by applying a first layer atop the substrate and comprising, in major weight part, a non-refractory first metal. A second layer is applied atop the first layer and comprises, in major weight part, a carbide and/or nitride of a second metal. A third layer is applied atop the second layer and comprises, in major weight part, a ceramic. The substrate may be a refractory metal-based investment casting core.

BACKGROUND OF THE INVENTION

The invention relates to metallic coating. More particularly, theinvention relates to protective coating of oxidizable investment castingcores.

Investment casting is a commonly used technique for forming metalliccomponents having complex geometries, especially hollow components, andis used in the fabrication of superalloy gas turbine engine components.

Gas turbine engines are widely used in aircraft propulsion, electricpower generation, and ship propulsion. In gas turbine engineapplications, efficiency is a prime objective. Improved gas turbineengine efficiency can be obtained by operating at higher temperatures,however current operating temperatures in the turbine section exceed themelting points of the superalloy materials used in turbine components.Consequently, it is a general practice to provide air cooling. Coolingis provided by flowing relatively cool air from the compressor sectionof the engine through passages in the turbine components to be cooled.Such cooling comes with an associated cost in engine efficiency.Consequently, there is a strong desire to provide enhanced specificcooling, maximizing the amount of cooling benefit obtained from a givenamount of cooling air. This may be obtained by the use of fine,precisely located, cooling passageway sections.

A well developed field exists regarding the investment casting ofinternally-cooled turbine engine parts such as blades and vanes. In anexemplary process, a mold is prepared having one or more mold cavities,each having a shape generally corresponding to the part to be cast. Anexemplary process for preparing the mold involves the use of one or morewax patterns of the part. The patterns are formed by molding wax overceramic cores generally corresponding to positives of the coolingpassages within the parts. In a shelling process, a ceramic shell isformed around one or more such patterns in well known fashion. The waxmay be removed such as by melting in an autoclave. The shell may befired to harden the shell. This leaves a mold comprising the shellhaving one or more part-defining compartments which, in turn, containthe ceramic core(s) defining the cooling passages. Molten alloy may thenbe introduced to the mold to cast the part(s). Upon cooling andsolidifying of the alloy, the shell and core may be mechanically and/orchemically removed from the molded part(s). The part(s) can then bemachined and treated in one or more stages.

The ceramic cores themselves may be formed by molding a mixture ofceramic powder and binder material by injecting the mixture intohardened steel dies. After removal from the dies, the green cores arethermally post-processed to remove the binder and fired to sinter theceramic powder together. The trend toward finer cooling features hastaxed core manufacturing techniques. The fine features may be difficultto manufacture and/or, once manufactured, may prove fragile.Commonly-assigned co-pending U.S. Pat. No. 6,637,500 of Shah et al.discloses general use of refractory metal cores in investment castingamong other things. Various refractory metals, however, tend to oxidizeat higher temperatures, e.g., in the vicinity of the temperatures usedto fire the shell and the temperatures of the molten superalloys. Thus,the shell firing may substantially degrade the refractory metal coresand, thereby produce potentially unsatisfactory part internal features.Also, the refractory metals may be subject to attack from components ofthe molten superalloys. Use of protective coatings on refractory metalcore substrates may be necessary to protect the substrates fromoxidation at high temperatures and/or chemical interaction with thesuperalloy. An exemplary coating involves first applying a layer ofchromium to the substrate and then applying a layer of aluminum oxide tothe chromium layer (e.g., by chemical vapor deposition (CVD)techniques). However, particular environmental/toxicity concerns attendthe use of chromium. Accordingly, there remains room for furtherimprovement in such coatings and their application techniques.

SUMMARY OF THE INVENTION

One aspect of the invention involves an investment casting corecomprising a coated refractory metal based substrate. A first coatinglayer consists principally (e.g., in major weight part) of a ceramic. Asecond coating layer is located between the first layer and thesubstrate and consists principally of one or more carbides and/ornitrides There is at least one of: a third layer located between thesecond layer and the substrate and consisting in major part of one ormore additional metals having an FCC lattice structure; and a solidsolution surface layer of the substrate having a minor amount of saidone or more additional metals.

In various implementations, the ceramic may consist essentially of atleast one of alumina, mullite, magnesia, and silica. The substrate maybe molybdenum-based. There may be no such third layer. The one or moreadditional metals may consist essentially of nickel. The first layer mayconsists essentially of aluminum oxide and the first thickness is anominal (e.g., median) first thickness. At a first location: the firstlayer may have a first thickness is at least 4.0μ; the second layer mayhave a second thickness of 1.0 4.0μ; and the substrate may have athickness in excess of 50μ. The core may be a first core in combinationwith: a ceramic or refractory metal second core; and a hydrocarbon-basedmaterial in which the first core and the second core are at leastpartially embedded.

Another aspect of the invention involves an article of manufacturecomprising a refractory metal-based substrate. A first means provides abarrier. A second means, located between the first means and thesubstrate, secures the first means and contains one or more carbidesand/or nitrides. A third means, located between the second means and thesubstrate, essentially prevents infiltration of at least one of carbonand nitrogen from the second means into the substrate. In variousimplementations, the first means may be ceramic, the second means may bea carbide, and the third means may be an fcc material.

Another aspect of the invention involves a method for coating asubstrate. A first layer is applied atop the substrate and comprises, inmajor weight part, a non-refractory first metal. A second layer isapplied atop the first layer and comprises in major weight part acarbide and/or nitride of a second metal. A third layer is applied atopthe second layer and comprises, in major weight part, a ceramic.

In various implementations, the first metal may be essentially diffusedinto the substrate, at least a major portion of which occurs during oneor both of the applying of the second layer and the applying of thethird layer. The ceramic may consist essentially of an oxide of a thirdmetal. The substrate may comprise, in major weight part, one or morerefractory metals. The first layer may be deposited directly atop thesubstrate. The second layer may be deposited directly atop the firstlayer. The third layer may be deposited directly atop the second layer.The first metal may form an FCC lattice structure. The second metal maybe titanium. The ceramic may consist essentially of at least one ofalumina, mullite, magnesia, and silica. The first layer may be depositedby electroplating. The second and third layers may be deposited by vapordeposition. The first layer may be deposited to a first thickness of atleast 1μ (e.g., 1–3μ). The second layer may be deposited to a secondthickness of least 0.5μ (e.g., 1–3μ). The third layer may be depositedto a third thickness of least 5μ (e.g., 15–25μ). The substrate mayconsist essentially of a molybdenum-based material. The method may beused to form an investment casting core component. The method mayfurther comprise: at least one of assembling the core with a second coreand forming a second core partially over the core; molding a sacrificialmaterial to the core and the second core; applying a shell to thesacrificial material; essentially removing the sacrificial material;casting a metallic material at least partially in place of thesacrificial material; and destructively removing the core, the secondcore, and the shell. The destructively removing may comprise essentiallyremoving at least the first layer and the second layer using HNO₃.

Another aspect of the invention involves a method for coating asubstrate. There is a step for applying a first layer for essentiallypreventing carbon infiltration into the substrate. There is a step forapplying a carbon-containing second layer for adherence with a thirdlayer. There is a step for applying the third layer as a barrier.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a shelled investment casting patternfor forming a gas turbine engine airfoil element.

FIG. 2 is a sectional view of a refractory metal core of the pattern ofFIG. 1.

FIG. 3 is a flowchart of processes for forming and using the pattern ofFIG. 1.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

FIG. 1 shows a shelled investment casting pattern 20 including a pattern22 and a ceramic shell 24. The pattern 22 includes a sacrificialwax-like material 26 (e.g., natural or synthetic wax or otherhydrocarbon-based material) at least partially molded over a coreassembly. The core assembly includes a ceramic feed core 28 having aseries of generally parallel legs 30, 32, and 34 for forming a series ofgenerally parallel, spanwise-extending, feed passageways in the ultimatepart being cast (e.g., a gas turbine engine turbine blade, or vane).Assembled to the feed core 28 are a series of refractory metal cores(RMCs) 36 and 38. Portions of the RMCs 36 and 38 may be received incompartments 40 and 42 in the feed core 28 and secured therein viaceramic adhesive 44. Other portions of the RMCs 36 and 38 may beembedded in the shell 24 so that the RMCs 36 and 38 ultimately formoutlet passageways from the feed passageways to the exterior surface ofthe part. The exemplary RMCs 36 provide film cooling passageways forairfoil pressure and suction side surfaces and the exemplary RMC 38provides airfoil trailing edge cooling. Many other configurations arepossible either in the prior art or yet to be developed.

FIG. 2 shows further details of one of the RMCs (e.g., 38). Theexemplary RMC 38 has a substrate 50 of refractory metal or a refractorymetal-based alloy, intermetallic, or other material. Exemplaryrefractory metals are Mo, Nb, Ta, and W. These may be obtained as wireor sheet stock and cut and shaped as appropriate. A coating systemincludes a base layer 52 initially deposited atop the substrate.Although shown discretely for purposes of illustration, in an exemplaryembodiment the base layer material becomes diffused into the substratematerial. An intermediate layer 54 is atop the base layer and an outerlayer 56 is atop the intermediate layer.

The exemplary outer (and outermost) layer 56 may provide a combinationof chemical protection, mechanical protection, and thermal insulation,(e.g., acting as a substantial barrier to infiltration of casting metalthat might alloy with or otherwise attack the substrate and to oxygen toprevent oxidation). Exemplary outer layer materials are ceramics(e.g.,aluminum oxide (alumina), mullite, silicon dioxide (silica), andmagnesium oxide (magnesia)) built up by deposition (e.g., chemical vapordeposition (CVD)).

The exemplary intermediate layer 54 may serve principally as a bondinglayer for good adherence of the outer layer 56. The intermediate layermay also provide a backup or additional barrier against oxygen.Exemplary intermediate layer materials are carbides or nitrides (e.g.,titanium carbide) built up by deposition (e.g., CVD). Such materials areadvantageously stable at outer layer deposition temperatures in therange of 1500–1600° C.

The exemplary base (and innermost) layer 52 may serve to at leasttemporarily secure the intermediate layer to the substrate while notadversely reacting with the substrate. Exemplary base layer materialscomprise metals having a face centered cubic (FCC) structure (e.g.,nickel or platinum) built up by electroplating. Such a lattice structuremay have advantageous tolerance for incidental infiltration of carbonand/or nitrogen atoms during deposition of the intermediate layerwithout either catastrophic loss of structural integrity or substantialtransmission of such atoms to the substrate. In the absence of such abase layer, in the elevated temperatures typical of CVD there would besubstantial infiltration of the carbon and/or nitrogen into thesubstrate. This infiltration may be particularly problematic with bodycentered cubic (BCC) lattice structure typical of refractory metals. Theinfiltration may form an embrittled layer containing the carbide and/ornitride of the refractory metal. This embrittlement may serve as asource of cracks propagating through the coating layers.

The exemplary substrate 50 is formed, e.g., from sheet stock having asurface including a pair of opposed faces 57 and 58 with a thickness Tbetween. Complex cooling features may be stamped, cut, or otherwiseprovided in the substrate 50. An interior surface 60 of the coatingsystem and base layer 52 sits atop the exterior surface of the substrate50 and an exterior surface 62 of the coating system and outer layer 54provides an exterior surface of the RMC 38. The transitions betweenlayers may be abrupt or may have compositional gradients. In theexemplary embodiment, the base layer 52 has an as-deposited thicknessT₂, the intermediate layer 54 has a thickness T₃, and the outer layer 56has a thickness T₄. Exemplary T is at least 50μ, more narrowly at least100μ. Exemplary T₂ is 1–10 μ, more narrowly, 1–4μ, or 1–3μ. Exemplary T₃is 0.5–5μ, more narrowly 1–4μ or 1–3μ. Exemplary T₄ is at least 4μ, morenarrowly 5–25μ, or 15–25μ.

FIG. 3 shows an exemplary process 200 of manufacture and use (simplifiedfor illustration) of the exemplary. The substrate(s) are formed 202 suchas via stamping from sheet stock followed by subsequent bending or otherforming to provide a relatively convoluted shape for casting the desiredfeatures. After any cleaning to remove residual oxides (e.g., acidand/or alkali wash followed by deionized water rinse), a first metal(e.g., essentially pure nickel) is applied 204 atop the substrate (e.g.,by electroplating) to form the base layer 52.

After any further cleaning, one or more carbides and/or nitrides of oneor more second metals (e.g., essentially pure titanium carbide, which iscommercially available at low cost) is applied 206 (e.g., by CVD) toform the intermediate layer. At the elevated temperatures of the CVDprocess, at the inboard Mo/Ni boundary, there may be interdifussion,creating a region of Mo-Ni solid solution. Also, small amounts of carbonmay diffuse into the nickel from the deposition vapor, especially at thebeginning of the deposition process, before substantial titanium carbideaccumulation. The ceramic barrier material (e.g., alumina) is applied210 (e.g., also by CVD in the same chamber immediately after titaniumcarbide deposition) to form the outer layer 56. During the deposition ofthe outer layer 56, the interdiffusion of the Mo and Ni may continue.Advantageously essentially all the Ni is consumed. The resulting solidsolution layer may have a relatively low nickel concentration (e.g., 2%or less at the outboard extreme). The absence of the Ni layer improvesthermal performance because of the relatively low melting temperature ofthe Ni. Such diffusion of the Ni has not been completed at the end ofdeposition, it may be achieved by a postdeposition heating step.Alternatively or additionally, a predeposition heating step may give thediffusion a partial head start. Additional layers, treatments, andcompositional/process variations are possible.

The RMC(s) are then assembled 220 to the feed core(s) or other core(s).Exemplary feed cores may be formed separately (e.g., by molding fromsilicon-based or other ceramic material) or formed as part of theassembling (e.g., by molding such feed core material partially over theRMC(s)). The assembling may also occur in the assembling of a die forovermolding 222 the core assembly with the wax-like material 26. Theovermolding 222 forms a pattern which is then shelled 214 (e.g., via amulti-stage stuccoing process forming a silica-based shell). Thewax-like material 26 is removed 216 (e.g., via steam autoclave). Theremay be additional mold preparation (e.g., trimming, firing, assembling).The firing may perform all or part of the postdeposition heating toensure Mo-Ni interdiffusion noted above. A casting process 218introduces one or more molten materials (e.g., for forming a superalloybased on one of more of Ni, Co, and Fe) and allows such materials tosolidify. The shell is then removed 220 (e.g., via mechanical means).The core assembly is then removed 222 (e.g., via chemical means). Theas-cast casting may then be machined 224 and subject to furthertreatment 226 (e.g., mechanical treatments, heat treatments, chemicaltreatments, and coating treatments).

The present system and methods may have one or more advantages overchromium-containing coatings. Notable is reduced toxicity. Chromiumcontaining coatings are typically applied using solutions of hexvalentchromium, a particularly toxic ion. Furthermore, when the coated core isultimately dissolved, some portion of the chromium will return to thistoxic valency. The present coatings may have less than 0.2%, preferablyless than 0.01% chromium by weight, and, most preferably, no detectablechromium.

One or more embodiments of the present invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention. Forexample, the coatings may be utilized in the manufacture of cores ofexisting or yet-developed configuration. The details of any suchconfiguration may influence the details of any particular implementationas may the details of the particular ceramic core and shell materialsand casting material and conditions. Accordingly, other embodiments arewithin the scope of the following claims.

1. An investment casting core comprising: a refractory metal-basedsubstrate; a first layer consisting principally of a ceramic; and asecond layer, located between the first layer and the substrate,consisting principally of one or more carbides and/or nitrides, whereinthere is at least one of: a third layer located between the second layerand the substrate and consisting in major part of one or more additionalmetals having an FCC lattice structure; and a solid solution surfacelayer of the substrate having a minor amount of said one or moreadditional metals.
 2. The core of claim 1 wherein: the ceramic consistsessentially of at least one of alumina, mullite, magnesia, and silica;the substrate is molybdenum-based.
 3. The core of claim 1 wherein: thereis no said third layer; and the one or more additional metals consistsessentially of nickel.
 4. The core of claim 1 wherein: the first layerconsists essentially of aluminum oxide and the first thickness is anominal first thickness.
 5. The core of claim 1 wherein at a firstlocation: the first layer has a first thickness is at least 4.0μ; thesecond layer has a second thickness of 1.0–4.0μ; and the substrate has athickness in excess of 50μ.
 6. The core of claim 1 being a first core incombination with: a ceramic or refractory metal-based second core; and ahydrocarbon-based material in which the first core and the second coreare at least partially embedded.